loadStyle method doesn't check style validity

[jjhembd]

The loadStyle method doesn't check the style at all. We need to confirm:

• If a link is supplied, is the linked document valid JSON?
• Does it follow the style specification?

Even for an invalid JSON, the error is not captured and returned

[jjhembd]

This is mostly addressed by 1d589f7. The Promise returned by the loadStyle method will reject in any of the following cases:

• Invalid URL
• HTTP errors
• JSON parsing error
• Style document is missing a required root property (version, sources, layers)

I think this is enough to resolve the issue for now. We might in the future want to add more detailed checks of the style against the specification.

Note: this does NOT resolve the relevant point in GlobeletJS/GlobeletJS#1, because the errors are not yet neatly passed through tile-setter